From: hupu <[email protected]>
I think the 'lag' calculation here is inaccurate.
Assume that delta needs to be subtracted from v_i to ensure that the
vlag of task i after placement is the same as before. At this time, the
vlag of task i after placement should be:
vl'_i = V' - (v_i - delta)
From the above formula, we know that vl'_i should be:
vl'_i = (vl_i * W)/(W + w_i)
That is to say:
V' - (v_i - delta) = (vl_i * W)/(W + w_i)
For a newly added entity, generally set v_i to V', and the above formula
can be converted into:
V' - (V' - delta) = (vl_i * W)/(W + w_i)
Therefore the value of delta should be as follows, where delta is the
'lag' in the code.
delta = (vl_i * W)/(W + w_i)
Signed-off-by: hupu <[email protected]>
---
kernel/sched/fair.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 03be0d1330a6..c5f74f753be8 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5239,9 +5239,12 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
if (curr && curr->on_rq)
load += scale_load_down(curr->load.weight);
- lag *= load + scale_load_down(se->load.weight);
+ lag *= load;
+
+ load += scale_load_down(se->load.weight);
if (WARN_ON_ONCE(!load))
load = 1;
+
lag = div_s64(lag, load);
}
--
2.17.1
On Tue, Apr 23, 2024 at 07:44:16PM +0800, [email protected] wrote:
> From: hupu <[email protected]>
>
> I think the 'lag' calculation here is inaccurate.
>
> Assume that delta needs to be subtracted from v_i to ensure that the
> vlag of task i after placement is the same as before.
Why ?!? v_i is the unkown, it makes no sense to complicate things by
adding extra unknowns.
> At this time, the
> vlag of task i after placement should be:
> vl'_i = V' - (v_i - delta)
But but but, you can't have V' without knowing v_i.
> From the above formula, we know that vl'_i should be:
> vl'_i = (vl_i * W)/(W + w_i)
>
> That is to say:
> V' - (v_i - delta) = (vl_i * W)/(W + w_i)
>
> For a newly added entity, generally set v_i to V', and the above formula
> can be converted into:
> V' - (V' - delta) = (vl_i * W)/(W + w_i)
>
> Therefore the value of delta should be as follows, where delta is the
> 'lag' in the code.
> delta = (vl_i * W)/(W + w_i)
>
> Signed-off-by: hupu <[email protected]>
> ---
> kernel/sched/fair.c | 5 ++++-
> 1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 03be0d1330a6..c5f74f753be8 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5239,9 +5239,12 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
> if (curr && curr->on_rq)
> load += scale_load_down(curr->load.weight);
>
> - lag *= load + scale_load_down(se->load.weight);
> + lag *= load;
> +
> + load += scale_load_down(se->load.weight);
> if (WARN_ON_ONCE(!load))
> load = 1;
> +
> lag = div_s64(lag, load);
You're making it:
v_i = V - (W * vl_i) / (W + w_i)
In direct contradiction to the giant comment right above this that
explains why the code is as it is.
> }
>
> --
> 2.17.1
>
On Thu, Apr 25, 2024 at 07:44:59PM +0800, [email protected] wrote:
> > > From: hupu <[email protected]>
> > >
> > > I think the 'lag' calculation here is inaccurate.
> > >
> > > Assume that delta needs to be subtracted from v_i to ensure that the
> > > vlag of task i after placement is the same as before.
> >
> > Why ?!? v_i is the unkown, it makes no sense to complicate things by
> > adding extra unknowns.
> >
> > > At this time, the
> > > vlag of task i after placement should be:
> > > vl'_i = V' - (v_i - delta)
> >
> > But but but, you can't have V' without knowing v_i.
> >
>
> Thank you for your patient guidance. I overlooked a important fact that
> v_i is unknown in the process of proof. Below is the complete proof
> process, and it turns out that you are correct.
*phew*, thanks for checking!